Shape memory alloys are metallic substances that significantly change their geometry when undergoing a phase transformation as their temperature is altered. Typically, the shape memory effect results from a phase transformation, which occurs at a specific temperature. In some cases, the alloy undergoes a transition from martensite (at lower temperatures) to austenite (at higher temperatures). If the metal is drawn to a wire, the length of such a wire can change significantly as the temperature of the metal is changed. One of the first shape memory alloys was a combination of nickel and titanium called Nitinol that was developed at the U.S. Naval Ordinance Laboratory in the 1960s. Many other metallic combinations exhibiting shape memory effects have since been developed. Some of the features of such alloys that are useful are control of the change in length with temperature, and obtaining a reproducible change in length or shape over a plurality of thermal cycles. The original shape is recovered when the temperature returns to the original value. In the form of wires, many shape memory alloys will shorten in length when heated beyond the phase transition temperature, often by more than 5% of the wire length. Wires are commercially sold in various sizes and are fairly inexpensive (typically a few dollars/meter). One commercial source of shape memory alloy wires is Dynalloy, Inc., 3194-A Airport Loop Drive, Costa Mesa, Calif. 92626-3405 (see www.dynalloy.com).
Elastomers are rubber like polymers that can often be stretched to several times their original dimension. They can also absorb appreciable compressive strain before yielding. In recent years soft-lithography has been used to make micro-fluidic devices using these polymers. Soft-lithography involves pouring the uncured polymer over a mold and then curing it, for example with heat or UV radiation. See for example “From micro- to nanofabrication with soft materials,” Quake, S. R. & Scherer, A. (Nov. 24, 2000) Science, 290, 1536-1540. The elastomer can then be peeled off the mold and this can be used to create two dimensional structures like channels. By aligning and bonding two layers of these elastomers a pneumatic valve may be built. See, for example, “Monolithic Microfabricated Valves and Pumps by Multilayer Soft Lithography,” Marc A. Unger, Hou-Pu Chou, Todd Thorsen, Axel Scherer, and Stephen R. Quake (7 Apr. 2000) Science 288 (5463), 113. [DOI: 10.1126/science.288.5463.113]) Because the elastomers can be molded to nanometer sizes highly dense fluidic devices can be built. See for example “Microfluidic Large-Scale Integration,” Todd Thorsen, Sebastian J. Maerki, and Stephen R. Quake (26 Sep. 2002) Science [DOI: 10.1126/science.1076996])
Microfluidic structures and systems are useful for many purposes, including such applications as chemical and biochemical sensing systems, protein and chemical synthesis, and spectroscopy of ultra-small volumes. However, in conventional microfluidic systems, pneumatic pressure lines with external macroscopic valves have to be used to actuate the components such as valves and pumps that control or perform various microfluidic functions that need to occur to make the processes work.
There is a need for miniaturized valves useful in microfluidic systems, for example to permit such systems to be reduced in size and weight.